Beyond D10, D50, and D90, the span of the particle size distribution is a critical parameter that determines how the powder will pack and flow.
Definition: Span = (D90 - D10) / D50It represents the width of the distribution relative to the median. A lower span means a tighter, more uniform particle size.
Typical MIM powder span values:| Powder Type | Typical Span | Interpretation |
|---|---|---|
| Gas atomized (narrow cut) | 1.2-1.8 | Tight distribution — uniform particles, predictable behavior |
| Water-gas combined atomized | 1.5-2.2 | Moderate distribution — good balance of packing and flow |
| Water atomized (as-atomized) | 2.0-3.0 | Broad distribution — includes both fines and coarse particles |
| Bimodal (engineered blend) | > 3.0 | Two distinct populations — designed for maximum packing density |
| Span Range | Packing Density | Feedstock Viscosity | Sintering Behavior | Recommendation |
|---|---|---|---|---|
| < 1.5 | Lower — uniform particles leave more interstitial space | Lower viscosity (less friction) | More uniform shrinkage | Good for thin-wall parts requiring high flowability |
| 1.5-2.5 | Higher — fine particles fill gaps between coarse particles | Higher viscosity (more particle interactions) | More consistent densification | Best general range for production MIM |
| > 2.5 | Highest possible packing density | Highest viscosity | Less shrinkage, potential segregation | Requires careful compounding to avoid PSD segregation during handling |
Span is calculated as (D90 - D10) / D50 and describes the width of the particle size distribution. A lower span (1.2-1.8) means tight, uniform particle sizes — good flow but lower packing density. A higher span (2.0-3.0) means a broad distribution that packs more densely but may flow less freely. The optimal span for MIM is typically 1.5-2.5, balancing packing density with flowability.